The present invention relates generally to sewage systems, and more particularly, to sewage systems which utilize differential air pressure to create flow therein in contrast to conventional gravity-operated and positive pressure sewer systems.
Typical gravity-operated systems include a network of underground pipes that provide continuous downhill flow of sewage to be collected at appropriate termination points. Oftentimes a termination point is an intermediate pumping station from which sewage is pumped to a treatment facility for appropriate processing. The pumping stations, or lift stations, are utilized in order to avoid deeply burying the pipes required for a long pipe run beneath flat or irregular terrain. Many gravity sewer systems thus incorporate mechanical pumps and as a consequence are not entirely operated by gravity forces. Generally, the pipes used in gravity systems are at least eight inches in diameter or larger.
Positive pressure sewage systems do not require pipes to be laid out so as to conform to topographic features. Positive pressure systems utilize a pump to direct sewage under pressure into a collection line which feeds to an intermediate station. The pumping station then feeds a sewage treatment plant. Systems of this type use plastic pipe having smaller diameters than the vitreous tile or concrete pipe used in gravity systems. Positive pressure sewage systems also require pressure pumps to be located at every sewage input point.
As an alternative to the conventional gravity-operated and the positive pressure sewage systems of the prior art, various types of vacuum sewage systems have been proposed. U.S. Pat. No. 3,115,148 issued to S. A. J. Liljendhal describes a vacuum system for separately conveying waste products discharged from watercloset bowls, urinals, and like sanitary apparatus, while the waste products, or gray water, from bathtubs, wash basins, sinks, and the like are conveyed by a separate conventional gravity system. The waste product conduits according to the Liljendhal patent are provided with "pockets" in which sewage is collected so as to form a plug which entirely fills the cross-sectional area of the conduit. A plug of sewage is moved by a pressure differential force along a conduit in an integral condition. Note that systems according to the Liljendhal patent have two separate piping systems for transporting waste products and gray water.
U.S. Pat. No. 3,730,884 issued to B. C. Burns, et al. describes a sewage system which uses "vacuum-induced plug flow." A system according to that patent handles both waste products and gray water with one piping system. In order for a system of this type to operate, a "coherent plug" of sewage is transported by a vacuum pressure differential through a pipe for a short distance. As the plug moves through the pipe, friction and other forces cause the plug to disintegrate with the result that the pressure differential moving the plug quickly diminishes. After breakdown of the coherent plug, the formation of a new coherent plug is aided by a plug reformer which in simplest form may be a dip, or pocket, in the pipe which serves as a trap for sewage. The pockets are designed so that sewage entirely fills the pipe bore and system operation is predicted on having a plug of sewage seal the pipe bore. A system may have several pockets located at various points along a pipe. When a new plug of sewage material is first injected into such a system, the sewage plugs already contained in the pockets in the pipe are moved through the pipe. As a plug moves through the pipe it disintegrates and the remains thereof flow by gravity to nearby pockets wherein new coherent sewage plugs are reformed. This process of alternate plug disintegration and reforming continues until the sewage eventually passes completely through the pipe. Note that the available pressure differential for each of these plugs is less than the total available system pressure differential available because of the serial arrangement of the plug pockets in a pipe.